Composite tubes made with either 304L, A825 or A625 exterior layers are commonly used to
construct the lower-furnace waterwalls of black liquor recovery boilers currently operated in North
America. While these composite tubes have been very successful in resisting sulphidation corrosion,
they have experienced unanticipated problems with the exterior layer, namely cracking, and, to a lesser
extent, preferential corrosion of port opening tubes. In an attempt to evaluate the suitability of
alternative composite tube alloys, a laboratory-based testing program was conducted to evaluate the
corrosion resistance of ferritic stainless steels (Types 430 and 446) and nickel-based alloys (Alloys 600
and 690) in simulated black liquor recovery boiler lower-furnace environments. An effort was made to
utilize simulated environments that are believed to reproduce the key features of the various damage
modes typically observed in the lower-furnace sections of high-pressure recovery boilers. This report
documents the major findings of this effort.
Keywords: recovery furnaces; corrosion; stress corrosion; cracks; sulfidation; composites; tubes;
stainless steel; alloy